Green’s Dictionary of Slang

Oliver’s skull n.

[originating in the Restoration’s hatred of the Commonwealth’s Lord Protector, Oliver Cromwell (1599–1658)]

a chamberpot.

[UK]B.E. Dict. Canting Crew n.p.: Oliver’s skull a Chamber-pot.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[US]Matsell Vocabulum.